The drive towards near-zero emissions fossil fuel technologies, to reduce environmental pollution, is opening up many new opportunities for research and development innovation and technology development in the area of particulate and emissions control. While conventional electrostatic precipitators have been found effective at removing particulates in relatively low temperature (less than 300° C.) flue gas streams, very little progress has been made on using electrostatic precipitators (ESPs) to remove particulates in hot (above 300° C.) gaseous streams, typically from fossil fuel combustion and gasification synthetic gas generation processes. Hot gas particulate clean up is one area which offers opportunities to significantly reduce both operating and capital cost in many solid and liquid fossil fired process configurations.
The need to reduce particulate concentrations is common to all coal-based fuels, many fossil liquid fuels and other energy conversion technologies. Such particulate concentrations generally include silica, sulphur and nitrogen-containing species, alkali metals, halogens and trace heavy metals. In a Brayton cycle, the removal of particles, alkali and salts from the flue gas is necessary to minimize the damage to the turbine components and eliminate, or reduce so far as possible, severe corrosion and erosion problems. In a Rankine steam cycle, the removal of particulate matter from hot flue gas streams would permit the boiler to be designed with improved heat transfer surfaces, thereby greatly improving the performance and reducing the maintenance and operating cost of the boiler, and would also reduce the erosion and particulate loading on down-stream flue gas systems.
ESPs are used to separate particles from carrier gas by the application of an electrostatic charge. Conventional ESPs, which comprise electrically charged plates, operate up to about 300° C. and are not capable of capturing very fine and submicron particles. The effectiveness of charging a particle is inversely proportional to the square of the diameter of the particle. Due to large inter-electrode spacing, fine particles are not charged well in conventional ESPs and hence they can escape easily without being captured. Moreover, at temperatures above 300° C. the resistivity of particulates is greatly reduced, making conventional ESP technology less practical. Extremely large collection plates would have to be used; making the technology cumbersome and prohibitively expensive. Although there are some technologies to clean flue gas below 300° C., only a few technologies are commercially used to clean hot flue gases above 300° C.
Recently, ESPs have been developed in which the conventional charged plates are replaced by sieving screens, comprising a mesh in a plane perpendicular to the gaseous flow direction, the mesh having suitably sized apertures through which the flow passes towards a grounded collection area, typically a second screen. Such sieving screens can operate in a gaseous flow at temperatures above 300° C. For example, U.S. Pat. No. 6,878,192 to Pasic discloses an ESP primarily for use in a combustion flue gas stream, to replace conventional plates or baghouse filters. In the ESP disclosed in U.S. Pat. No. 6,878,192, the gaseous flow travels through a series of screens, in which single electrically charged screens are alternated with single grounded screens. It has been found that it is advantageous for each electrically charged screen to be in close proximity to the succeeding grounded screen, and that it is generally advantageous to alternate the electrically charged screens with the grounded screens. The reference does not address the complex issues of arrangement of the alternating polarity screens, nor issues relating to spatial relationships or cleaning, nor the additional issues which arise when substantially higher temperatures are involved than those contemplated by the reference, as discussed further below.
Conventionally, the electrically charged screens are provided with a negative charge, but it is known that positively charged screens can also be used with some beneficial effect. It is further known to enhance the effect of the charging screens by providing for a corona discharge by means of spikes provided to a first sieving screen, protruding into the gaseous flow, which results in an advantageous corona discharge, and hence enhanced particulate recovery.
Conventionally, the particulate matter accumulates on the grounded plates of ESPs, which are cleaned periodically at suitable intervals from the plates by an active process such as rapping or vibration, or by sweeping their surfaces by a mechanical means. Various methods of regulating the cleaning process for electrostatic precipitators using plates are known. For example, WO 2008/109592 suggests a method of controlling the order of rapping for the electrode plates of different sections. Similarly, for sieving screens, a rapping process can also be used. However, the timing intervals for these known processes in relation to the various screens may be quite arbitrary and consequently allow for excessive accumulation on some of the screens. Also, inappropriate rapping of two or more screens at the same time tends to result in particulate re-entrainment in the flue gas stream and carry through, which reduces the capture effectiveness of the precipitators.
It is therefore desirable to develop an electrostatic precipitator that does not suffer from these disadvantages noted above, and further disadvantages, which render the known ESPs less suitable or completely unsuitable for many applications.
It is particularly desirable to develop a method of using a sieving ESP to segregate particulate species, such as gaseous sulphur and nitrogen-containing species, alkali metals, halogens and trace heavy metals, contained in hot flue gas streams associated with combustion and gasification (either in air or oxygen-enriched environments), i.e. at temperatures above 300° C.